PI3K Activation, Inhibition, & Medical Implications

Activation of Class IA PI3K
Inactive PI3Ks are rapidly activated in the presence of extracellular stimuli. Such stimuli, as discussed previously, include growth factor receptors with intrinsic protein tyrosine kinase activity, which display pYXXM motifs for p85 docking, as well as receptor substrates which are phosphorylated and interact with PI3K regulatory subunits like nSH2. PI3K can be additionally activated in cooperative processes like translocation to the plasma membrane where lipid substrates are available and by binding GTP loaded Ras to the catalytic subunit.

PI3K Inhibition


Inhibition by Wortmannin, LY294002 & Others: Implications
Wortmannin is an irreversible inhibitor of PI3-Kinases by alkylating a lysine residue at the putative ATP binding site of p110. LY294002 is a competitive inhibitor of ATP. Due to their instability and lack of selectivity leading to toxicity, neither wortmannin nor LY294002 are not valid pharmaceutical therapeutics. That being said, these compounds along with Quercetin, Myricetin and Staurosporine, can serve as excellent tools for investigating PI3K structure. Further, derivatives of wortmannin with more favorable pharmacological profiles are currently in clinical trials for various PI3K associated diseases.

Wortmannin Binding
Wortmannin binds the p110 kinase domain ATP-binding site, positioning itself in a conserved pocket using conserved p110α residues Ile 800, Ile 848, Val 850, Val 851, Ser 919, Met 922, Phe 930, Ile 932 and Asp 933. Wortmannin forms a covalent bond with Lys 802 and hydrogen bonds with Asp 933, Tyr 836, Val 851 and Gln 859 Inhibition of the ATP binding site prevents binding of ATP and subsequent transfer of the γ-phosphate group of ATP to PIP2.

LY294002, Quercetin, Myricetin & Staurosporine
LY294002, a competitive inhibitor of ATP binding in the PI3K kinase domain, was first discovered by scientists at Eli Lilly. Quercetin, Myricetin & Staurosporine are natural compounds which broadly inhibit protein kinases. Understanding how ATP binds to the ATP binding site within the kinase domain of PI3Kγ and how various inhibitors prevent this interaction helps elucidate ways to develop effective, selective inhibitors. See p110γ bound to ATP (1e8x), Wortmannin (1e7u), LY294002 (1e7v), Quercetin (1e8w), Staurosporine (1e8z), Myricetin (1e90).



The Phosphorylated Lipid Products in Downstream Signaling
Ligand receptor interactions trigger a rapid rise of cellular PIP3. Numerous molecular targets are activated upon interaction with PIP3. One such target is the Ser/Thr kinase Akt, which requires the action of phosphoinositide dependent kinases, another step for potential fine tuning. Akt subsequently inactivates glycogen-synthase-kinase 3 and the pro-apoptotic factor BAD. . PIP3 also activates Btk, an essential protein for normal B lymphocyte development and function along with dozens of other targets including centaurin, profiling, cytohesin, etc. which control

Medical Implications


PI3K In Medicine
As mentioned previously, the class I PI3Ks play a critical role in the transmission of proliferation and survival signals in a wide variety of cell types. Due to PI3Ks intricate activation system by numerous targets, mutations at key positions in PI3K have been identified to cause various types of cancer. These positions are known as “Hotspots.” These hotspots are located in both the p85 subunit and p110 subunit. For example, a mutation in the nSH2 domain known to cause glioblastoma is G376R. <scene name='User:David_Canner/Sandbox_P/Med_376/1'>Gly 376 is at hydrogen bonding distance of Glu 365 a crucial residue in one of the stabilizing C2 domain loops. Somatic mutations in the gene encoding the p110 catalytic subunit can be grouped into the four classes of the catalytic subunit in which they occur, the ABD, C2, helical and catalytic domains, all of which likely increase PI3K activity by different mechanisms. For example, two well known cancer causing mutations map to <scene name='User:David_Canner/Sandbox_P/Helical_abd_out/2'>the ABD, at residues <scene name='User:David_Canner/Sandbox_P/Helical_abd_mutations/1'>Arg 38, Arg 88. These residues lie at the interface of the ABD and Kinase domains and are believed to alter regulation of the catalytic subunit. Other mutations, such as those in the C2 domain, up regulate PI3K, by increasing the affinity for substrate containing membranes, resulting in elevated levels of PIP3. Aberrations in PIP3 levels, either through activation of PI3Ks or through inactivation of lipid phosphatase PTEN, occur frequently in numerous forms of cancer. Recent data suggest that at least 50% of human breast cancers involve mutations in either PI3K or PTEN.

The dramatic number of mutations in PI3K associated with Cancer has resulted in PIK3CA, the gene that encodes the catalytic p100( domain of PIK3, being identified as a human oncogene. More than 1500 PIK3CA mutations, nearly all of which increase lipid kinase activity, have been identified in different tumor types, the most common being breast and uterine cancers. Further, the lipid products of PI3K interact with other well known oncogenes like akt2, akt3, PDGFR, PTEN, among many others.

In addition to cancer, faulty PI3K function has been associated with disorders like heart failure, diabetes, , and inflammation.

Current Pharmaceutical Approaches
Broad spectrum PI3K inhibitors have exhibited impressive results, revealing increased apoptosis and decreased proliferation in tumor models. The primary focus now amongst medicinal researchers is to identify PI3K inhibitors with increased selectivity (particularly for p110) and bioavailability. Use of inhibitors such as wortmannin have identified slightly different binding mechanisms between PI3K isoforms, creating the potential for highly selective compounds to neutralize secific PI3K isotypes while leaving other forms of the ubiquitous protein unaltered.PI3K stands as one of the most promising targets for pharmaceutical intervention of cancer. </StructureSection>

Additional Resources

 * See Phosphoinositide 3-Kinases for the main page or The Structure of PI3K for information on PI3K's structure and function.
 * See Cancer for additional information.
 * See Diabetes for additional information.